2011-08-18_HYDROLOGY - M1977306

• Permit No: M1977306
• IBM Index Class Name: HYDROLOGY
• Doc Date: 8/18/2011
• Doc Name: Colorado Discharge Permit System
• From: Water Quality Control Division
• To: Cotter Corporation
• Email Name: GRM
• Media Type: D
• Archive: No

COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division
Rationale — Page 9, Permit No. CO0036251
c. Determination of Total Maximum Daily Loads (TMDLs) — The Dolores River is on the 303(d)
list of water quality impacted streams for total recoverable iron. Since the discharge is not
expected to reach the Dolores River from any of the outfalls, monitoring for iron will not be
required at this time.
d. Colorado Mixing Zone Regulations — Pursuant to section 31.10 of The Basic Standards and
Methodologies for Surface Water, a mixing zone determination is required for this permitting
action. The Colorado Mixing Zone Implementation Guidance, dated April 2002, identifies the
process for determining the meaningful limit on the area impacted by a discharge to surface
water where standards may be exceeded (i.e., regulatory mixing zone). This guidance document
provides for certain exclusions from further analysis under the regulation, based on site - specific
conditions.
The guidance document provides a mandatory, stepwise decision - making process for
determining if the permit limits will not be affected by this regulation. Exclusion, based on
Extreme Mixing Ratios, may be granted if the ratio of the design flow to the chronic low flow
(30E3) is greater than 2:1 or if the ratio of the chronic low flow to the design flow is greater than
20:1. Since the direct receiving stream is a zero low flow stream, the exemption has been met
and no further analyses are required under the regulation.
e. Agricultural Use Protection — Section 31.11(1)(a)(iv) of The Basic Standards and Methodologies
for Surface Waters (Regulation No. 31) includes the narrative standard that State surface waters
shall be free of substances that are harmful to the beneficial uses or toxic to humans, animals,
plants, or aquatic life.
As stated in Appendix A, the water in the unnamed tributaries of East Paradox Creek and. Bull
Canyon is not used for irrigation.
f. Reasonable Potential Analysis — Using the assimilative capacities contained in the WQA, an
analysis must be performed to determine whether to include the calculated assimilative capacities
as WQBELs in the permit. This reasonable potential (RP) analysis is based on the Determination
of the Requirement to Include Water Quality Standards -Based Limits in CDPS Permits Based on
Reasonable Potential, dated December, 2002. This guidance document utilizes both quantitative
and qualitative approaches to establish RP depending on the amount of available data.
A qualitative determination of RP may be made where ancillary and/or additional treatment
technologies are employed to reduce the concentrations of certain pollutants. However, absent
limitations, a facility may no longer continue such pollutant reductions and therefore the
discharge would result in RP. For this reason, the Permits Section may make a qualitative
determination that absent effluent limitations, there is RP for these pollutants to cause or
contribute to exceedances of water quality standards.
A qualitative RP determination may also be made where a Federal ELG exists for a parameter,
and where the results of a quantitative analysis results in no RP. As the Federal ELG is typically
less stringent than a limitation based on the WQBELs, if the discharge was to contain
concentrations at the ELG (above the WQBEL), the discharge may cause or contribute to an
exceedance of a water quality standard.